This invention concerns a process for producing premium gasoline by polymerizing C.sub.4 cuts, then adding value to C.sub.4 olefinic cuts available in large amounts on the market as a result of the developing processes of naphtha and gas oils steam-cracking and catalytic cracking.
Olefins, butenes and isobutenes may be used in petrochemical syntheses of more elaborate products (alcohols, aldehydes, acids, nitriles, etc.), but the available amounts of these C.sub.4 cuts are generally too large or at least may become too large for having their use limited to said first way of adding value to the C.sub.4 cuts.
A second way of adding value to a C.sub.4 cut consists of recycling this C.sub.4 cut to the cracking unit (steam-cracking or catalytic cracking unit) after hydrogenation of the olefins; but here, the isobutane contained in the hydrogenated cut, not exceeding 50% by weight of said cut, does not give sufficiently high yields to ethylene and also results in a substantial production of methane which cannot be used to a large extent, except as fuel.
A third way of adding value to a C.sub.4 cut consists, after hydrogenation of said cut, of separating the n-butane from the isobutane by distillation and recycling to the steam-cracking unit only the n-butane, while keeping the isobutane for other more advantageous uses, such as for producing, for example, isoparaffinic gasolines by alkylation. However, the ethylene and propylene yields, in such a steam-cracking, do not exceed 38 and 20% respectively, by weight, and the methane production is about 25% by weight.
A fourth way of adding value to a C.sub.4 cut consists of alkylating the olefins of said cut with the isobutane contained therein, so as to obtain a maximum yield of gasolines. However, in this alkylation step, the isobutene behaviour is less favorable than that of the butenes for obtaining a gasoline of high octane number; as a matter of fact, the Research (or Clear) octane numbers of normal butenes are higher than those of isobutene alkylates.
It still exists other ways, for example, that consisting of converting the isobutene of the charge to isobutene di- and trimers without substantially modifying the butenes of the charge, of subsequently isomerizing the 1-butene of the charge to 2-butene in view of subsequently subjecting the 2-butene to an alkylation reaction with isobutane (U.S. Pat. No. 4,392,002).
Another method would consist in a modification of this latter technique according to which the totality of the butenes present in the initial charge are converted to isobutene dimers and trimers, which products form an excellent motor fuel.
This method is performed (see, for example, the French Pat. No. 2,515,171), by subjecting the charge, (olefinic C.sub.4 cut), to a polymerization during which the isobutene is converted, in a proportion of 90% by weight or more, to dimers and trimers, which are separated from the unconverted butenes by mere fractionation and fed to the gasoline pool. The normal butenes (unreacted or having only reacted to a small extent during the polymerization), are then separated and the C.sub.4 paraffinic hydrocarbons extracted therefrom can be fed to the gasoline pool. The fraction subjected to extraction has a very high content of n-butenes (at least 99% by weight) and those n-butenes are converted by isomerization to isobutene which, in turn, will be converted to isobutene dimers and trimers.
By this way, the totality of the initial olefinic C.sub.4 cut can be converted to gasoline.